The present application claims a priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 2001-373684 filed on Dec. 7, 2001, entitled xe2x80x9cInformation recording medium and method for producing the samexe2x80x9d. The contents of that application are incorporated herein by the reference thereto in their entirety.
1. Field of the Invention
This invention relates to an information recording medium which is used for optically or electrically recording, erasing, overwriting and reproducing information, and a method for producing the same.
2. Description of Related Art
The inventors developed 4.7 GB DVD-RAM which is a large capacity rewritable phase-change type information recording medium and can be used as a datafile and an image file. This has been already commercialized.
This 4.7 GB DVD-RAM is disclosed, for example, in Japanese Patent Kokai (Laid-Open) Publication No. 2001-322357. The constitution of DVD-RAM disclosed in this publication is shown in FIG. 9. The information recording medium 31 shown in FIG. 9 has a seven-layer structure where a first dielectric layer 102, a first interface layer 103, a recording layer 4, a second interface layer 105, a second dielectric layer 106, an optical compensation layer 7, and a reflective layer 8 are formed on one surface of a substrate 1 in this order. In this information recording medium, the first dielectric layer exists in a position closer to an incident laser beam than the second dielectric layer. The same relationship exists between the first interface layer and the second interface layer. Thus, in this specification, when the information recording medium contains two or more layers having the same function, xe2x80x9cfirstxe2x80x9d xe2x80x9csecondxe2x80x9d xe2x80x9cthirdxe2x80x9d . . . is given to the beginning of the name of each layer in the order of the layer which is closer to the incident laser beam.
The first dielectric layer 102 and the second dielectric layer 106 have the function which adjusts an optical path length so as to enhance the optical absorption efficiency of the recording layer 4, and enlarges the difference between the reflectance of crystal phase and the reflectance of amorphous phase so as to enlarge a signal amplitude. ZnS-20 mol % SiO2 (i.e. (SiO2)80(ZnS)20) conventionally used as a material for the dielectric layer is amorphous material. It has low thermal conductivity, is transparent, and has a high refractive index. Moreover, ZnS-20 mol % SiO2 exhibits a high film-forming speed at the time of the film formation, and good mechanical characteristic and moisture resistance. Thus, ZnS-20 mol % SiO2 is an excellent material suitable for forming the dielectric layer.
If the thermal conductivity of the first dielectric layer 102 and the second dielectric layer 106 is low, the heat can diffuse from the recording layer 4 to the reflective layer 8 quickly in the thickness direction when a laser beam enters the recording layer 4, and therefore, in-plane heat diffusion in the dielectric layers 102 or 106 is suppressed. That is, the recording layer 4 is cooled by the dielectric layer for a shorter time, and therefore, an amorphous mark (record mark) can be easily formed. When a record mark is hard to form, a high peak power is necessary for recording. When a record mark is easy to form, recording can be conducted with a low peak power. When the thermal conductivity of the dielectric layer is low, recording can be conducted with a low peak power, and therefore, the recording sensitivity of the information recording medium becomes higher. On the other hand, when the thermal conductivity of the dielectric layer is high, recording is conducted with a high peak power, and therefore the recording sensitivity of the information recording medium becomes lower. The dielectric layer in the information recording medium exists in a form of such thin film that thermal conductivity cannot be measured accurately. For this reason, the inventors employ the recording sensitivity of the information recording medium as a relative judgment reference for learning the degree of the thermal conductivity of the dielectric layer.
The recording layer 4 is formed using the material containing Gexe2x80x94Snxe2x80x94Sbxe2x80x94Te which crystallizes at a high speed. The information recording medium which contains such material as the recording layer 4, not only has excellent initial recording characteristic, but also has excellent archival characteristic and an excellent archival overwrite characteristic. In a phase-change type information recording medium, information is recorded, erased and overwritten by utilizing reversible phase change between crystal phase and amorphous phase of the recording layer 4. When the recording layer 4 is irradiated with a high power (i.e. peak power) laser beam, and then cooled rapidly, the irradiated part turns into an amorphous phase and a record mark is formed. When the recording layer is irradiated with a low power (i.e. bias power) laser beam to raise its temperature and then cooled gradually, the irradiated part turns into a crystal phase and recorded information is erased. By irradiating the recording layer with the laser beam of which power is modulated between the peak power level and the bias power level, it is possible to overwrite new information while erasing information already recorded. Overwrite cyclability is expressed with the maximum number which corresponds to repeatable overwrite number on the condition that the jitter value does not cause a problem in a practical use. It can be said that the better overwrite cyclability is, the larger this number is. Particularly, an information recording medium for datafiles is expected to have excellent overwrite cyclability.
The first interface layer 103 and the second interface layer 105 have the function which prevents a material transfer caused between the first dielectric layer 102 and the recording layer 4, and between the second dielectric layer 106 and the recording layer 4, respectively. The material transfer here means the phenomenon which S of ZnS-20 mol % SiO2 of the first and second dielectric layers diffuses into the recording layer while the recording the layer is irradiated with a laser beam and information is repeatedly overwritten. If a lot of S diffuses into the recording layer, a reduction of the reflectance of the recording layer is caused, and overwrite cyclability deteriorates. This phenomenon has already been known (See N. Yamada et al. Japanese Journal of Applied Physics Vol.37 (1998) pp.2104-2110). Moreover, Japanese Patent Kokai (Laid-Open) Publication No. 10-275360 and International Publication No. WO 97/34298 disclose that the interface layer which prevents this phenomenon is formed using a nitride containing Ge.
The optical compensation layer 107 adjusts the ratio Ac/Aa where Ac is optical absorptance of the recording layer 4 in a crystalline state, and Aa is optical absorptance Aa of the recording layer 4 in an amorphous state, and serves to suppress distortion of overwritten marks. The reflective layer 8 optically serves to increase the light quantity absorbed by the recording layer 4, and thermally serves to diffuse the heat generated in the recording layer 4 to cool the recording layer quickly and to facilitate amorphization of the recording layer. The reflective layer 8 also serves to protect a multilayered film from the operation environment.
Thus, the information recording medium shown in FIG. 9 ensures excellent overwrite cyclability and high reliability with a large capacity of 4.7 GB by using the structure including the seven layers each of which functions as mentioned above, and thereby has been commercialized.
As material suitable for the dielectric layer of the information recording medium, various materials have already been proposed. For example, in Japanese Patent Kokai (Laid-Open) Publication No. 5-159373, it is disclosed that the heat-resistance protective layer is formed from a mixture of at least one compound selected from nitride, carbide, oxide and sulfide with a melting point higher than that of Si, and low alkali glass in an optical information recording medium. In this publication, the carbide, oxide, and sulfide of Nb, Zr, Mo, Ta, Ti, Cr, Si, Zn, and Al, are illustrated as the high melting point compound. Moreover, in the publication, it is disclosed that the low alkali glass essentially consists of SiO2, BaO, B2O3, and Al2O3.
As mentioned above, when forming the first and the second dielectric layers by using ZnS-20 mol % SiO2, the interface layer is inevitably needed between the recording layer and the dielectric layer for preventing the diffusion of S. However, when considering price of the medium, it is desirable that the number of the layers which compose the medium is as small as possible. If the number of layers is small, reduction of the cost of materials, miniaturization of manufacturing apparatus, and the increase in the throughput due to reduction in manufacture time can be realized, which results in the reduction of the price of the medium.
The inventors examined a possibility of eliminating at least one of the first interface layer and second interface layer as one method of reducing the number of layers. The inventors considered that in this case, a dielectric layer needs to be made from material other than ZnS-20 mol % SiO2 so that the diffusion of S from the dielectric layer into the recording layer due to overwriting may not be caused. Further, the followings are desired as to the material for the dielectric layer:
The adhesiveness of the material to the recording layer which is of chalcogenide material is good;
The material realizes that recording sensitivity which is equivalent to or higher than that of the above seven-layer structure;
The material is transparent; and
The material has a high melting point so that it may not melt when recording.
It is an object of the present invention to provide an information recording medium which is provided with a dielectric layer having favorable adhesiveness to a recording layer, in which medium a substance does not transfer from the dielectric layer to the recording layer even when the dielectric layer is formed in direct contact with the recording layer without forming the interface layer, and excellent overwrite cyclability is ensured.
The above-mentioned Japanese Patent Kokai (Laid-Open) Publication No. 5-159373 does not refer to the problem that a substance transfers from the dielectric layer to the recording layer. Therefore, it should be noted that this publication does not teach the problem which this invention solves, and means to solve the problem, i.e. a specific composition of the material for the dielectric layer.
The inventors formed the dielectric layer by using various compounds and evaluated the adhesiveness of the dielectric layer to the recording layer and overwrite cyclability of the information recording medium, as explained in the below-mentioned Example. As a result, it was found that, when providing a dielectric on both sides of the recording layer directly, without an interface layer, the adhesiveness of the dielectric layer to the recording layer is good in the case where the dielectric layer is formed from a material which is easy to diffuse in the recording layer, for example, the conventional ZnS-20 mol % SiO2, although, overwrite cyclability of the medium is inferior. Moreover, for example, ZrO2 has low thermal conductivity and a high melting point. Therefore, if ZrO2 is used for a dielectric layer, the recording sensitivity of the information recording medium can be high and excellent overwrite cyclability can be ensured. However, when forming a dielectric layer using ZrO2, the result was that the adhesiveness of the dielectric layer to the recording layer is inferior. With respect to the information recording medium in which the dielectric layer is formed in contact with the recording layer using other various oxides, nitrides, sulfides and selenides, the adhesiveness of the dielectric layer to the recording layer and overwrite cyclability were evaluated. However, when forming the dielectric layer using one kind of oxide, nitride, sulfide or selenide, favorable adhesiveness and favorable overwrite cyclability could not be obtained together.
The inventors examined forming a dielectric layer from a mixture of ZnS and a compound other than SiO2 which is mixed with ZnS in the conventional ZnS-20 mol % SiO2. As a result, it has been found that a combination of ZnS and ZrO2 is suitable as a constitutive material for the dielectric layer which contacts with the recording layer, which led to this invention.
That is, the present invention provides an information recording medium which includes a substrate and a recording layer wherein a phase change between a crystal phase and an amorphous phase is generated by irradiation of light or application of an electric energy, and which further includes a Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer comprising Zr, Zn, S and O. Here, the term xe2x80x9cmaterial layer comprising Zr, Zn, S and Oxe2x80x9d is used based on the intention that it does not matter in what compound each atom exists. In this material, however, it is considered that most Zr exists in the form of ZrO2 with O, while Zn and S exist in the forms of ZnS, Zn which does not bond to another atom, and S which does not bond to another atom. As mentioned below, in this specification, the combination of the three forms of ZnS, Zn which does not bond to another atom, and S which does not bond to another atom, are regarded as forming a system which is indicated as xe2x80x9cZnxe2x80x94S.xe2x80x9d The characteristic of the material layer comprising Zr, Zn, S and O is that the adhesiveness of the layer to the recording layer is good and the material is difficult to transfer between this layer and the recording layer.
The information recording medium of the present invention is a medium on or from which information is recorded or reproduced by irradiation of light or by application of an electric energy. Generally, irradiation of light is carried out by irradiation of a laser light (that is, laser beam), and application of an electric energy is carried out by applying a voltage to a recording layer. Hereafter, the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer which constitutes the information recording medium of this invention is described in detail.
More specifically, the information recording medium of this invention includes the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer which consists essentially of the material expressed with the formula (1):
ZrBZnCSDO100xe2x88x92Bxe2x88x92Cxe2x88x92D (atomic %)xe2x80x83xe2x80x83(1) 
wherein B, C and D are respectively within the range of 8xe2x89xa6Bxe2x89xa633, and 3xe2x89xa6Cxe2x89xa630, Cxe2x89xa6Dxe2x89xa62Cxe2x89xa645, and satisfy 40xe2x89xa6B+C+Dxe2x89xa680, as a constituent element. The xe2x80x9catomic %xe2x80x9d here shows that the formula (1) is a compositional formula of which basis (i.e. 100%) is the sum of the numbers of Zr, Zn, S and O atoms. Also in the following formulae, the indication of xe2x80x9catomic %xe2x80x9d is used for showing the same meaning.
In the formula (1) it does not matter what compound each atom of Zr, Zn, S and O forms. The reason why the material is specified by this formula is that it is difficult to determine the composition represented with ratio of each compound when analyzing the composition of a layer formed into a thin film, and actually only an elementary composition (that is, ratio of each atom) is often determined. As mentioned in the above, in the material expressed with the formula (1), it is considered that most Zr exists as ZrO2 with O, and most Zn exists as Znxe2x80x94S with S. Here, xe2x80x9cZnxe2x80x94Sxe2x80x9d means that, in the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer, not only ZnS exists, but also Zn which does not bond to another atom and/or S which does not bond to another atom exist. Therefore, Znxe2x80x94S is indicated as, for example, ZnS1.5. The indication shows that (the number of S atoms)/(the number of Zn) is 1.5. Further, in this case, an excess of S which does not bond to Zn (or another atom) exists in the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer. In the formula (1), since C and D satisfy Cxe2x89xa6Dxe2x89xa62Cxe2x89xa645, (the number of S atoms)/(the number of Zn atoms) is in the range of 1 to 2.
Preferably, the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer which consists essentially of the material expressed with the above-mentioned formula (1) exists as either dielectric layer of the two dielectric layers adjacent to the recording layer in the information recording medium. More preferably, it exists as both of the two dielectric layers. In the dielectric layer which contains Zr, Zn, S and O in the above-mentioned range, ZrO2 with a melting point higher than 2700xc2x0 C. ensures excellent overwrite cyclability and Znxe2x80x94S ensures adhesiveness to the recording layer which is of chalcogenide material. Further, a thin film of ZrO2 is amorphous and a thin film of ZnS is crystalline. When two materials whose structure is different from each other are mixed to make complex the structure of the mixture, the thermal conductivity of the mixture is lowered. Thereby, the recording layer is cooled more rapidly, and therefore, the recording sensitivity of the information recording medium is improved. Therefore, in the information recording medium of the present invention, delamination between the recording layer and the dielectric layer is not caused even when the interface layer is not provided, and good overwrite cyclability and recording sensitivity are exhibited. Alternatively, the layer of the material expressed with the formula (1) may be an interface layer which is located between the recording layer and a dielectric layer in an information recording medium.
The Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer substantially consisting of the material expressed with the formula (1) may be the layer which substantially consists of the material expressed with the formula (11):
(ZrO2)X(Znxe2x80x94S)100xe2x88x92X (mol %)xe2x80x83xe2x80x83(11) 
wherein X is in the range of 50xe2x89xa6Xxe2x89xa680. The formula (11) expresses the preferable ratio of the two compounds when the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material consists of ZrO2 and Znxe2x80x94S. The term xe2x80x9cmol %xe2x80x9d here shows that the formula (11) is a compositional formula of which basis (i.e. 100%) is the total of each compound. Also in the following formulae, the indication of xe2x80x9cmol %xe2x80x9d is used for showing the same meaning.
Preferably, the layer which substantially consists of the material expressed with the formula (11) also exists as either dielectric layer of the two dielectric layers adjacent to the recording layer. More preferably, it exists as both of the two dielectric layers. The effect by using the layer which substantially consists of the material expressed with the formula (11) as a dielectric layer is the same as described in relation to the material expressed with the formula (1). The content of ZrO2 is preferably 50 mol % or more in order to ensure good recording sensitivity of the information recording medium. On the other hand, the content of ZrO2 is preferably 80 mol % or less in order to ensure the adhesiveness brought by Znxe2x80x94S. Alternatively, the layer which substantially consists of the material expressed with the formula (11) may be an interface layer which is located between the recording layer and a dielectric layer in an information recording medium.
In the information recording medium of this invention, the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer may further contains Si, and substantially consist of the material expressed with the formula (2):
ZrESiFZnGSHO100xe2x88x92Exe2x88x92Fxe2x88x92Gxe2x88x92H (atomic %)xe2x80x83xe2x80x83(2) 
wherein E, F, G and H are respectively in the range of 1xe2x89xa6Exe2x89xa630, 0xe2x89xa6Fxe2x89xa623, 2xe2x89xa6Gxe2x89xa630 and Gxe2x89xa6Hxe2x89xa62Gxe2x89xa645, and satisfy 40xe2x89xa6E+F+G+Hxe2x89xa680.
Also in the formula (2), it does not matter what compound each atom of Zr, Si, Zn, S and O forms. The reason why the material is specified by this formula is the same as the reason for employing the formula (1). In the material expressed with the formula (2), it is considered that most Si exists as SiO2 with O. Further, Gxe2x89xa6Hxe2x89xa62G means that (the number of S atoms)/(the number of Zn atoms) is in the range of 1 to 2.
Preferably, the layer which substantially consists of the material expressed with the formula (2) exists as either dielectric layer of the two dielectric layers adjacent to the recording layer. More preferably, it exists as both of the two dielectric layers. In the information recording medium in which the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer containing Si is employed as the dielectric layer, favorable adhesiveness of the dielectric layer to the recording layer and excellent overwrite cyclability are ensured, and higher recording sensitivity is realized. It is considered that the higher recording sensitivity is realized because the thermal conductivity of the layer becomes low by containing Si. Alternatively, the layer substantially consisting of the material expressed with the formula (2) may be an interface layer which is located between the recording layer and a dielectric layer in an information recording medium.
The Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer containing Si may be the layer which substantially consists of the material expressed with the formula (21):
(ZrO2)Y(SiO2)Z(Znxe2x80x94S)100xe2x88x92Yxe2x88x92Z (mol %)xe2x80x83xe2x80x83(21) 
wherein Y and Z are respectively within the range of 20xe2x89xa6Yxe2x89xa670, and 10xe2x89xa6Zxe2x89xa650, and satisfy 50xe2x89xa6Y+Zxe2x89xa680. The formula (21) shows the preferable ratio of three compounds when the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer containing Si consists of a mixture of ZrO2, SiO2, and Znxe2x80x94S. Preferably, the layer which consists essentially of the material expressed with the formula (21) exists as either dielectric layer of the two dielectric layers adjacent to the recording layer. More preferably, it exists as both of the two dielectric layers. Alternatively, the layer substantially consisting of the material expressed with the formula (21) may be an interface layer which is located between the recording layer and a dielectric layer in an information recording medium.
When using the layer which substantially consists of the material expressed with the formula (21) as a dielectric layer, SiO2 serves to enhance the recording sensitivity of the information recording medium. When using this material, the content of SiO2 (i.e. Z in the formula (21)) is preferably 10 mol % or more. On the other hand, if the content of SiO2 is too high, the content of other components becomes low, and thereby the adhesiveness of the layer to the recording layer, and the overwrite cyclability of the information recording medium deteriorate. Therefore, the content of SiO2 is preferably 50 mol % or less. Further, by adjusting the content of SiO2 within the range of 10 to 50 mol %, the recording sensitivity can be adjusted. In the material expressed with the formula (21), the content of Znxe2x80x94S is preferably 20 mol % or more so as to ensure the adhesiveness to the recording layer, and preferably 50 mol % or less so as not to cause S to diffuse into the recording layer. Therefore, in the formula (21), it is preferable that 50xe2x89xa6Y+Zxe2x89xa680,
The material expressed with the formula (21) may contain ZrO2 and SiO2 at a substantially equal ratio. In this case, this material is expressed with the following formula (22):
(ZrSiO4)A(Znxe2x80x94S)100xe2x88x92A (mol %)xe2x80x83xe2x80x83(22) 
wherein A is within the range of 33xe2x89xa6Axe2x89xa667. ZrSiO4 is a complex oxide which contains ZrO2 and SiO2 at a substantially equal ratio. ZrSiO4 has a high melting point and a stable structure with stoichiometric composition. Preferably, the layer which substantially consists of the material expressed with the formula (22) exists as either dielectric layer of the two dielectric layers adjacent to the recording layer. More preferably, it exists as both of the two dielectric layers. In the formula (22), ZrSiO4 and Znxe2x80x94S exist in the layer at a suitable ratio by setting A into the range of 33xe2x89xa6Axe2x89xa667. Therefore, the dielectric layer which substantially consists of the material expressed with the formula (22) adheres to the recording layer well and ensures that the information recording medium has favorable recording sensitivity and favorable overwrite cyclability. When ZrO2 and SiO2 form ZrSiO4 which is a complex oxide, a thin film of ZrSiO4 is amorphous. As mentioned in the above, a thin film of ZnS is crystalline. When materials whose structure is different from each other are mixed to make complex the structure of the mixture, the thermal conductivity of the mixture is lowered. Thereby, the recording layer is cooled more rapidly, and therefore, the recording sensitivity of the information recording medium is improved. Thus, the layer which substantially consists of the material expressed with the formula (22) is excellent in adhesiveness to the recording layer, and ensures that the information recording medium has favorable recording sensitivity and favorable overwrite cyclability. Alternatively, the layer which substantially consists of the material expressed with the formula (22) may be an interface layer which is located between the recording layer and a dielectric layer in an information recording medium.
In the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer which constitutes the information recording medium of the present invention, it is preferable that the number of S atoms and the number of Zn atoms satisfy 1xe2x89xa6(the number of S atoms)/(the number of Zn atoms)xe2x89xa62. When the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer in which (the number of S atoms)/(the number of Zn atoms) is in this range is formed in contact with the recording layer in the production process of the information recording medium, the medium is excellent in adhesiveness of the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer to the recording layer and has favorable overwrite cyclability and favorable recording sensitivity. More preferably, (the number of S atoms)/(the number of Zn) is 1 or more. In that case, the adhesiveness of the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer to the recording layer is more improved. However, (the number of S atoms)/(the number of Zn atoms) is preferably 2 or less. When (the number of S atoms)/(the number of Zn atoms) exceeds 2, the content of S in the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer becomes high, whereby the overwrite cyclability of the information recording medium deteriorates due to the diffusion of S into the recording layer, in the case where this Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer is formed as a dielectric layer in contact with the recording layer.
In any one of the formulae (1), (11), (2), (21) and (22), (the number of S atoms)/(the number of Zn atoms) is in the range of 1 to 2. Therefore, the content of S is 33 atomic % or less in all materials expressed with these formulae, and is less than the content of S in the conventional (ZnS)80(SiO2)20 (mol %) (=Zn36.4S36.4Si9.0O18.2), that is, 36.4 atomic %. In other words, the materials expressed with the above formulae ensure that, by defining the ratio of the number of S atoms to the number of Zn atoms, it is ensured that the crystallization characteristic of the recording layer and overwrite cyclability of the information recording medium do not deteriorate while favorable adhesiveness of the layer (specifically the dielectric layer) formed from any one of the materials to the recording layer is maintained.
The information recording medium of the present invention preferably has a recording layer in which a phase change is generated reversibly. That is, the information recording medium of this invention is preferably provided as a rewritable information recording medium.
Specifically, the recording layer where a phase change occurs reversibly preferably contains any one material selected from Gexe2x80x94Sbxe2x80x94Te, Gexe2x80x94Snxe2x80x94Sbxe2x80x94Te, Gexe2x80x94Bixe2x80x94Te, Gexe2x80x94Snxe2x80x94Bixe2x80x94Te, Gexe2x80x94Sbxe2x80x94Bixe2x80x94Te, Gexe2x80x94Snxe2x80x94Sbxe2x80x94Bixe2x80x94Te, Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te and Sbxe2x80x94Te. Each of these is a rapid crystallization material. Therefore, when a recording layer is formed from these materials, it is possible to record information at a high density and a high transfer rate, and to obtain the information recording medium excellent in reliability (specifically archival characteristic or archival overwrite characteristic).
The information recording medium of this invention may have two or more recording layers. For example, such information recording medium has a single-sided dual-layer structure, in which two recording layers are formed on one surface of a substrate with a dielectric layer and an intermediate layer therebetween. As to the information recording medium of the single-sided dual-layer structure, information is recorded in two recording layers by irradiation of light from one side. By employing this structure, the recording capacity can be made large. Alternatively, an information recording medium of this invention may include a recording layer on both surfaces of a substrate.
In the information recording medium of this invention, it is desirable that the film thickness of the recording layer is 15 nm or less. If it exceeds 15 nm, the heat applied to the recording layer will diffuse in the planar direction, and will be difficult to diffuse in the thickness direction.
The information recording medium of this invention may have a constitution in which a first dielectric layer, a recording layer, a second dielectric layer, and a reflective layer are formed in this order on one surface of a substrate. The information recording medium of this constitution is a medium on which information is recorded by irradiation of light. In this specification, the xe2x80x9cfirst dielectric layerxe2x80x9d means the dielectric layer which is in the position closer to the incident light, and the xe2x80x9csecond dielectric layerxe2x80x9d means the dielectric layer which is in the position farther from the incident light. That is, the incident light passes through the first dielectric layer and the recording layer in this order, and then reaches the second dielectric layer. The information recording medium of this constitution is used, for example, when recording and reproducing by the laser beam of which wavelength is about 660 nm.
When the information recording medium of this invention has this constitution, at least one of the first dielectric layer and the second dielectric layer is the above Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer (specifically, the layer which consists essentially of any one of the materials expressed with the above formulae (1), (11), (2), (21), and (22)). Preferably, both of the dielectric layers are the above Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer. In this case, the compositions of both dielectric layers may be the same or different from each other.
The information recording medium of this invention may have a constitution in which a reflective layer, a second dielectric layer, a recording layer, and a first dielectric layer are formed in this order on one surface of a substrate. This constitution is employed when the thickness of the substrate to which a light is applied needs to be thin. Specifically, the information recording medium of this constitution is used, when information is recorded and reproduced by a short-wavelength laser beam of which wavelength is about 405 nm, and the numerical aperture NA of an objective lens is made as large as, for example, 0.85 in order to set a focal position shallow. In order to use such a wavelength and numerical aperture NA, the thickness of the substrate to which light is applied needs to be set at between about 60 and 120 xcexcm, for example. It is difficult to form a layer on the surface of such thin substrate. Therefore, the information recording medium of this constitution is identified as an medium formed by using a substrate to which a light is not applied as a support substrate, and stacking a reflective layer and so on in the order on one surface of the substrate.
When the information recording medium of this invention has this constitution, at least one of the first dielectric layer and the second dielectric layer is the above Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer. Preferably, both of the dielectric layers are the above Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layers. In this case, the compositions of both dielectric layers may be the same or different from each other.
This invention also provides a method for producing the information recording medium of this invention which includes the process of forming the above-mentioned Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer by a sputtering method. According to the sputtering method, the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based layer of which composition is substantially the same as that of a sputtering target, can be formed. Therefore, according to this producing method, the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer of a desired composition can be easily formed by selecting a sputtering target appropriately.
Specifically, a sputtering target which substantially consists of the material expressed with the following formula (10):
ZrbZncSdO100xe2x88x92bxe2x88x92cxe2x88x92d (atomic %)xe2x80x83xe2x80x83(10) 
wherein, b, c, and d are respectively within the range of 11xe2x89xa6bxe2x89xa630, 5xe2x89xa6cxe2x89xa627 and cxe2x89xa6dxe2x89xa62cxe2x89xa640, and satisfy 40xe2x89xa6b+c+dxe2x89xa680, can be used. The formula (10) is equivalent to an elementary composition formula of the material expressed with the formula (110) mentioned below. Therefore, by using this target, the layer which substantially consists of the material expressed with the above-mentioned formula (10) can be formed.
The elementary composition of the layer formed by sputtering may differ from the elementary composition of a sputtering target depending on a sputtering device, sputtering conditions, and the size of the sputtering target and so on. Even when such difference occurs upon using the sputtering target consisting of the material expressed with the above-mentioned formula (10), the elementary composition of the layer to be formed is expressed at least with the above-mentioned formula (1).
In the producing method of the information recording medium of this invention, a sputtering target which substantially consists of the material expressed with the formula (110):
(ZrO2)x(Znxe2x80x94S)100xe2x88x92x (mol %)xe2x80x83xe2x80x83(110) 
wherein x is in the range of 50xe2x89xa6xxe2x89xa680, can be used. This is equivalent to the formula which expresses the composition of a sputtering target with the ratio of ZrO2 and Znxe2x80x94S. The reason why the sputtering target is thus specified is that the sputtering target consisting of the material which contains Zr, Zn, S and O is usually provided with the indication of the composition based on these two compounds. Further, the inventors have confirmed that, according to analysis with an X-ray microanalyser, the elementary composition of a commercially available sputtering target becomes substantially equal to the elementary composition calculated from the indicated composition (that is, the indicated composition (i.e. nominal composition) is correct). Therefore, this sputtering target makes it possible to form the layer which substantially consists of the material expressed with the formula (11).
In the producing method of the information recording medium of this invention, in order to form the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer containing Si, a sputtering target which substantially consists of the material expressed with the formula (20):
ZreSifZngShO100xe2x88x92exe2x88x92fxe2x88x92gxe2x88x92h (atomic %)xe2x80x83xe2x80x83(20) 
wherein e, f, g and h are respectively in the range of 3xe2x89xa6exe2x89xa627, 1xe2x89xa6fxe2x89xa620, 5xe2x89xa6gxe2x89xa627 and gxe2x89xa6hxe2x89xa62gxe2x89xa640, and satisfy 40xe2x89xa6e+f+g+hxe2x89xa680, may be used. When this sputtering target is used, the layer which substantially consists of the material expressed with the formula (21) or the formula (2) is formed.
In the producing method of the information recording medium of this invention, a sputtering target which substantially consists of the material expressed with the formula (210):
(ZrO2)y(SiO2)z(Znxe2x80x94S)100xe2x88x92yxe2x88x92z (mol %)xe2x80x83xe2x80x83(210) 
wherein y and z are respectively within the range of 20xe2x89xa6yxe2x89xa670, and 10xe2x89xa6zxe2x89xa650, and satisfy 50xe2x89xa6y+zxe2x89xa680, may be used. The reason why the sputtering target is thus specified is that the sputtering target consisting of the material containing Zr, Zn, Si, S and O is usually provided with the indication of composition based on ZrO2, SiO2, and Znxe2x80x94S. The inventors have confirmed that also the indicated composition (i.e. nominal composition) of the target whose composition is indicated with the formula (210) is correct. Therefore, this sputtering target makes it possible to form the layer which substantially consists of the material expressed with the formula (21).
The sputtering target expressed with the above-mentioned formula (210) may contain ZrO2 and SiO2 at a substantially equal ratio. In that case, the sputtering target which substantially consists of the material expressed with the formula (220):
(ZrSiO4)a(Znxe2x80x94S)100xe2x88x92a (mol %)xe2x80x83xe2x80x83(220) 
wherein a is within the range of 33xe2x89xa6axe2x89xa667, may be used. This sputtering target makes it possible to form the layer which consists of the material shown by the formula (22).
In any one of the above formulae (10), (110), (20), (210) and (220), (the number of S atoms)/(the number of Zn) is in the range of 1 to 2. Therefore, the sputtering target which consists of any one of the materials expressed with these formulae makes it possible to form the Zrxe2x80x94Znxe2x80x94Sxe2x80x94O-based material layer wherein (the number of S atoms)/(the number of Zn) is in the range of 1 to 2.
Preferably, this invention is characterized in that the dielectric layer is formed from ZrO2xe2x80x94(Znxe2x80x94S)-based material, ZrO2xe2x80x94SiO2xe2x80x94(Znxe2x80x94S)-based material, or the ZrSiO4xe2x80x94(Znxe2x80x94S)-based material, in direct contact with the recording layer. According to this characteristic, by eliminating the interface layer which is provided between the recording layer and the dielectric layer in the prior art optical information recording medium, the number of layers can be reduced, while the optical information recording medium which has high reliability and ensures excellent overwrite cyclability and high recording sensitivity, can be realized. When the layer of any one of these materials is used as a dielectric layer for insulating a recording layer in the information recording medium to which an electric energy is applied, the phase change of the recording layer can be generated with a small electric energy.